A lithium-ion secondary battery formed by making an electrode group by stacking sheet-shaped positive electrode and negative electrode with a separator interposed therebetween and housing this electrode group in a closed container made of metal or resin and filled with an electrolyte, the lithium-ion secondary battery provided with an external terminal connected to both electrodes of the electrode group, has been widely known.
In the above-described secondary battery, when the electrode group generates heat due to abnormal operations, a large amount of gas is sometimes generated, resulting in an increase in battery internal pressure. The abnormal operations include an overcharge, the application of heat, an internal short-circuit caused by an external load, and the like.
In the past, a gas release valve has been provided in a battery container to prevent an explosion of the battery container by discharging the gas from the gas release valve when the container internal pressure increases. However, there is a possibility that the gas release valve is obstructed as a result of the electrode group being moved by the container internal pressure at the time of the generation of gas.
In PTL 1, in a secondary battery in which a gas release valve is provided on the side of a container, the cross-sectional area (the first cross-sectional area) of the gap between an electrode group and the side of the container is made greater than the cross-sectional area (the second cross-sectional area) of the gap between the electrode group and the bottom face of the container so that the gas release valve is not obstructed even when the electrode group is moved by the gas generated from an electrode body.